In recent years, an integrated circuit using an SOI (silicon on insulator) substrate in which a thin single crystal semiconductor layer is formed on an insulating surface, instead of a bulk silicon wafer has been developed. Since parasitic capacitance between a drain of a transistor and a substrate is reduced by using the SOI substrate, the SOI substrate has attracted attention as one improving performance of semiconductor integrated circuits.
One of known methods for manufacturing SOI substrates is a hydrogen ion implantation separation method (for example, see Patent Document 1). A method for manufacturing an SOI substrate using a hydrogen ion implantation separation method is briefly described below. First, hydrogen ions are implanted into a silicon wafer by an ion implantation method to form a microbubble layer at a predetermined depth from the surface. Then, the silicon wafer into which the hydrogen ions are implanted is bonded to another silicon wafer with a silicon oxide film interposed therebetween. Then, by heat treatment, the microbubble layer serves as a cleavage plane and a part of the silicon wafer into which hydrogen ions have been implanted is separated in a thin film shape along the microbubble layer. Accordingly, a single crystal silicon film can be formed over the other bonded silicon wafer.
A method for forming a single crystal silicon layer over a base substrate made from glass by using such a hydrogen ion implantation method as described above has been proposed (for example, see Patent Document 2). Since a glass substrate has a larger area and is less expensive than a silicon wafer, when a glass substrate is used as a base substrate, an inexpensive large-area SOI substrate can be manufactured.
Further, in Patent Document 2, it is disclosed that a silicon nitride film is provided between a base substrate and a single crystal silicon layer so as to prevent impurities included in the base substrate or the like from diffusing into the single crystal silicon layer.